This invention relates to certain flame retardant polyamides and more particularly to such polyamides where the polyamides are copolymers of polyamide 6.6.
The use of melamine and melamine derivatives such as melamine cyanurate as flame retardant agents for polyamides has been known for many years (U.S. Pat. No. 3,660,344 and U.S. Pat. No. Re 30,402). In the case of polyamide 6.6, which is specifically used for its better thermal aging properties, versus polyamide 6, this has not been achieved with melamine because its volatility creates a large amount of melamine white mold deposition. Additionally, blooming of melamine from molded pieces under various simulated use conditions has been noted previously (U.S. Pat. No. 4,525,505) as a reason why this art has not been practiced commercially. The volatility of melamine and its tendency to sublime at typical polyamide 6.6 molding temperatures (270.degree. C.-290.degree. C.) has generally rendered melamine-containing polyamide 6.6 compositions unattractive from a commercial point of view. The alternative to melamine has been the use of melamine derivatives or its condensation products such as melamine cyanurate; but, although these have presented solutions to the melamine blooming issues, they have proved difficult to scale-up to typical commercial processes without some loss of flammability control.
The object of the invention is to provide unreinforced flame retardant polyamide molding compositions which have melamine as the flame retardant and which consistently meet the UL94 V-O criteria at all the specified thicknesses according to standards described for commercial acceptability based on statistical representation whilst at the same time minimizing any blooming of melamine during the molding process without reducing the thermal resistance properties of the polyamides.